1. Field of the Invention
This invention relates to the removal of aluminum from aluminosilicate compositions, especially from crystalline aluminosilicate zeolites. In a particular aspect, this invention is related to increasing the silica to alumina mole ratio of aluminosilicates.
2. Description of the Prior Art
Considerable effort has been devoted to the improvement of natural and synthetic aluminum-containing oxides such as clays, gels, aluminosilicates and the like with regard to pore structure, crystalline form, catalytic and adsorptive activity and thermal, hydrothermal and acid stability. Substantial incentive for this endeavor results from the broad application that such compositions have found as adsorbents, catalysts and ion exchange media in numerous chemical and hydrocarbon conversion and process systems. It is often the case that the chemical, physical and catalytic properties of these materials deteriorate on exposure to process environments, particularly those involving elevated temperatures, acidic process media or contact with steam. For example, the crystallinity and catalytic activity of aluminosilicate hydrocracking catalysts are known to deteriorate on exposure to repeated regeneration which often involves exposing the catalyst to an extreme temperature capable of burning off deposited coke and residual hydrocarbon. The regeneration of such cracking catalysts generally requires that the catalyst be able to withstand steam and/or thermal atmospheres at temperatures in a range of 1,300.degree.-1,700.degree. F. These compositions must also be able to withstand the chemically degrading influence of acidic materials such as the sulfur and nitrogen oxides encountered during regeneration of some systems.
The crystalline aluminosilicate zeolites represent a class of porous refractory inorganic oxides in which there is currently considerable interest in these and other respects. One factor that is known to influence the tolerance of crystalline aluminosilicates to acidic and thermal environments is the structural silica to alumina mole ratio. For any given class of aluminosilicates, catalytic activity, thermal stability and resistance to acid and steam attack are known to improve as the structural SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio is increased. The value of procedures that are effective in increasing this ratio is therefore readily apparent.
In synthetic crystalline aluminosilicate zeolites, the silica to alumina mole ratio is essentially determined by the nature of the starting materials and the relative quantities of such materials used in the preparation of the zeolite. Some variation in the silica to alumina mole ratio can be obtained by changing the proportion of reactants, e.g., increasing the relative concentration of the silica precursor relative to the alumina precursor. However, definite limits in the maximum obtainable silica to alumina mole ratio are observed. For example, synthetic faujasites having a silica to alumina mole ratio of about 5.2 to 5.6 can be obtained by increasing the relative proportion of the silica precursor. However, when the silica proportion is increased to even higher levels no commensurate increase in the silica to alumina mole ratio of the crystallized synthetic faujasite is observed. Thus, the silica to alumina mole ratio of about 5.6 must be considered an upper limit in a preparative process using conventional reagent. Corresponding upper limits in the silica to alumina mole ratio of mordenite and erionite via the synthetic pathway are also observed.
Accordingly, attempts to increase the silica to alumina mole ratio of crystalline zeolites by removal of aluminum from the crystal structure with strong acids have heretofore been reported. Further, the silica to alumina mole ratio of zeolites have been increased by converting the parent zeolite at least partially to its hydrogen form, hydrolyzing the aluminum to aluminum hydroxide, and thereafter physically removing the displaced aluminum.
U.S. Pat. No. 3,442,795 describes a process for preparing highly siliceous zeolite-type materials from crystalline aluminosilicates by means of a solvolysis technique, e.g. hydrolysis, followed by a chelation technique. In this disclosure, the acid form of a zeolite, e.g. zeolite Z, is subjected to a solvolysis technique such as hydrolysis, to remove aluminum from the aluminosilicate. The aluminum can then be physically separated from the aluminosilicate by the use of complexing or chelating agents such as ethylenediaminetetraacetic acid or carboxylic acid, to form aluminum complexes which are readily removable from the aluminosilicate. Ultra high silicon-content zeolites and preparation thereof are disclosed in U.S. Pat. No. 4,093,560.
A method of abstracting structural aluminum from aluminosilicates, thereby increasing the structural silica to alumina mole ratio while maintaining the ordered internal crystalline structure of the aluminosilicate is disclosed in U.S. Pat. No. 3,691,099. In said patent, the aluminosilicate is contacted with a mildly acidic solution containing at least one water soluble salt of a cation exchangeable with said aluminosilicate at a sufficient temperature and for a sufficient amount of time.
In U.S. Pat. No. 3,937,791, a method is described for removing alumina from a crystalline aluminosilicate. This method comprises heating said aluminosilicate to a temperature in the range between about 50.degree. C. and 100.degree. C. in the presence of a cationic form of chromium in an aqueous solution of above 0.01 Normal of a chromium salt of a mineral acid whereby the pH is less than 3.5, and such that the atomic ratio of chromium to aluminum is greater than 0.5.
A method for increasing the silica to alumina mole ratio of a crystalline aluminosilicate zeolite by contacting said zeolite with water at elevated temperature and then treating to remove alumina from the crystal lattice is disclosed in U.S. Pat. No. 3,591,488. Following the high temperature water treatment, amorphous alumina is removed from the zeolite material by contacting with a dilute mineral acid or an organic acid chelating agent.
In U.S. Pat. No. 3,640,681, framework aluminum is extracted from crystalline zeolites using acetylacetone as the extracting agent. Prior to contact with the acetylacetone, the zeolite must be rendered substantially cation-deficient and at least partially dehydroxylated. Other metals can be substituted for the extracted framework aluminum by contacting the zeolite with a metal acetylacetone.
The treatment of zeolites with gaseous chlorine compounds such as Cl.sub.2 or HCl to remove AlCl.sub.3 is described in Ger. Offen. No. 2,510,740.